1. Field of the Invention
The present invention relates to a multiplexing digital communications system, and more particularly to a multiplexing device having a digital 1-link relay capability for preventing the deterioration of voice quality or a delay of data, when multiplexing devices are connected via a digital relay exchange with multiple links.
2. Description of the Related Art
Currently, a multiplexing device for lowering the bit rate of a coded voice signal or a fax signal by performing data conversion is frequently used, for example, in an in-house dedicated line network so as to reduce a line cost, etc.
When a voice system network is configured by using a dedicated line, etc., the line is effectively used by coding and multiplexing a voice signal (a coded voice signal is hereinafter referred to as a coded signal) on some occasions. If the scale of such a network becomes large, an exchange may sometimes serve as a relay exchange, which relays a voice signal to a different point. When a connection is made via such a relay exchange, a non-reversible data conversion system may be sometimes used as a system for coding/decoding a voice signal. Therefore, the coding/decoding of a voice signal is repeated by the number of times that the voice signal is relayed by a relay exchange. As a result, the voice quality deteriorates. Additionally, a delay caused by repeating the encoding/decoding may become large. If only the multiplexing devices which are respectively closest to a call originating unit and a call terminating unit can be opposed to each other in order to overcome the above described problems, the coding and decoding operations are respectively performed only once. As a result, the deterioration of data and a delay can be reduced to a minimum. For example, a hearing difficulty in a voice transmission can be eliminated. As described above, the system for keeping the multiplexing device existing between the multiplexing devices, which are respectively closest to the call originating and terminating units, from performing the coding/decoding operations, and for directly relaying a coded signal to the multiplexing device closest to the call terminating unit via a digital relay exchange is referred to as a digital 1-link relay connecting system.
FIG. 1A is a block diagram showing the network configuration for explaining a digital 1-link relay capability. In FIG. 1A, for example, a voice signal, which is transmitted from an exchange A to an exchange C via a digital relay exchange B 100, is coded by a multiplexing device 101, propagated over a transmission line, decoded by a multiplexing device 102 arranged in the stage preceding the digital relay exchange B 100, and switched by the digital relay exchange B 100. Then, the signal is again coded by a multiplexing device 103, propagated over a transmission line, and decoded by a multiplexing device 104 arranged in the stage preceding the exchange C.
In such a network, a synchronous bit for relay exchange switching, which is referred to as an F bit to be described later, is inserted in, for example, the signal transmitted from the multiplexing device 102 to the relay exchange B 100. The multiplexing device 103 which has received the F bit via the digital relay exchange B 100 recognizes that it must only relay the voice signal by detecting the synchronization establishment with the F bit. The multiplexing device 103 therefore performs relay exchange mode operations for fundamentally outputting the signal transmitted from the digital relay exchange B 100 to the multiplexing device 104 side unchanged and not via its coder 103b, and for outputting the signal input from the multiplexing device 104 to the digital relay exchange B 100 side unchanged.
When the multiplexing device 102 detects the F bit which is inserted in the voice signal by the multiplexing device 103, it performs the relay exchange mode operations in a similar manner. That is, the multiplexing device 102 outputs the signal input from the digital relay exchange B 100 to the multiplexing device 101 unchanged, and also outputs the signal input from the multiplexing device 101 to the digital relay exchange B 100 unchanged. As a result, the voice signal input from the exchange A is coded only by the multiplexing device 101 and decoded only by the multiplexing device 104, thereby implementing the digital 1-link relay capability.
Provided below are the explanations about the configuration of a conventional multiplexing device, and the insertion of an F bit as the synchronous bit for relay exchange switching. FIG. 1B is a schematic diagram explaining the operations of a multiplexing device. This multiplexing device is hereinafter referred to as a conventional type (a). For example, a multiplexing device 102 is composed of a decoder 102a, a coder 102b, a bypass data transmitting unit 102c for fundamentally outputting a signal input from a transmission line to a digital relay exchange 100 side unchanged, and a bypass data receiving unit 102d for outputting the signal input from the digital relay exchange 100 side unchanged. F bit detection 102e operations are performed for the data input from the exchange side, while F bit insertion 102f operations are performed for the data output to the exchange side. The bypass data receiving unit 102d performs an in-phase operation due to the difference between the bit rates of a PCM (Pulse Code Modulation) signal and coded data, and performs control in order not to generate a lot of noise.
FIG. 1C is a schematic diagram showing the details of the configuration of the constituent elements of the multiplexing device 102 shown in FIG. 1B. Its configuration is almost the same as that of the multiplexing device 102 shown in FIG. 1B except for an adder 102f for inserting the F bit in a voice signal, which is arranged on the output side of the decoder 102a. 
FIG. 1D is a diagram explaining the method for inserting the F bit as the synchronous bit for relay exchange switching in a PCM voice signal. In FIG. 1D, an F bit is periodically inserted in a selected position among the LSBs (DOs) of the 8 bits (D7 through DO) of the voice signal on every 8 KHz clock cycle (cycle T=125 xcexcs).
FIG. 1E exemplifies a PCM signal after the switching to relay exchange mode is performed. Generally, xe2x80x9c1xe2x80x9d is assigned to an unused bit in order not to generate a lot of noise after the signal is converted into an analog signal while running in the relay exchange mode. Here, also all the values of the LSBs at the positions in which F bits are not inserted are xe2x80x9c1xe2x80x9d.
FIG. 1F is a diagram explaining the method for establishing synchronization with an F bit in the multiplexing device of the conventional type (a). In FIG. 1F, for example, one of 4 consecutive LSBs (Least Significant Bits) is used for an F bit after F bits are started to be inserted. Accordingly, 500 xcexcs is recognized to be one cycle of an F bit for a 64-Kbps voice signal (8 KHz clock). When a voice path is connected within a digital relay exchange and the voice data in which the F bit from the multiplexing device on an opposing side is inserted is input, the detection of a synchronous pattern, xe2x80x9c10xe2x80x9d in this case, is started. To improve the accuracy of the detection, the switching to the relay exchange mode is performed when 4 synchronous patterns are consecutively detected.
FIGS. 1G and 1H are diagrams for explaining the details of the method for inserting an F bit. In FIG. 1G, F bits are inserted in the positions xe2x80x9cbxe2x80x9d, xe2x80x9cfxe2x80x9d, xe2x80x9cjxe2x80x9d, xe2x80x9cnxe2x80x9d, . . . among the LSBs of voice data, and one of 4 LSBs is used for an F bit. Therefore, its cycle will become 500 xcexcs. xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d, . . . are inserted in the positions xe2x80x9cbxe2x80x9d, xe2x80x9cfxe2x80x9d, xe2x80x9cjxe2x80x9d, xe2x80x9cnxe2x80x9d, xe2x80x9crxe2x80x9d, . . . as F bits of actual LSBs.
FIG. 1H is a diagram explaining data arrangement of an actual PCM signal. FIG. 1H shows the case where a data clock is 64 KHz. The LSB of the initial 8 bits of the voice data corresponds to xe2x80x9caxe2x80x9d shown in FIG. 1G, and the LSB of the next 8 bits corresponds to xe2x80x9cbxe2x80x9d. xe2x80x9c0xe2x80x9d is inserted in the position xe2x80x9cbxe2x80x9d as an F bit. The reason that an F bit is assigned to an LSB is that an amplitude change is minimized when a signal is converted into an analog signal. When the F bit is inserted, only the data of the LSB is switched after serial/parallel conversion is performed.
A variety of methods can be considered as a method for detecting an F bit. By way of example, the method using a random access memory may be cited. That is, the method for preparing the memory having an F bit cycle width, sequentially storing data, and searching for F bits when the data is stored up to a certain number for protection. If this method is used, the memory for-performing a search, etc. is required. With this method, however, it is easy to find where F bits exist. There is a possibility that the actual data having the same cycle and pattern as those of an F bit is stored. Accordingly, the detection of only one synchronous pattern is not determined to be synchronization establishment. The detection of the synchronization establishment is determined when the number of identical synchronous patterns, the number of which is equal to a synchronization establishment detection number, are detected.
The reason that the synchronization establishment detection number is used is: synchronization can be possibly caused in a pseudo manner, because a voice signal, etc. may have random values even if a synchronous pattern having a certain string is defined to be one set and only one set is detected. Therefore, the detection of the synchronization establishment is not determined when only one pattern is detected. Namely, the determination of the synchronization establishment is protected and made by detecting several synchronous patterns. This prevents a detection error, which leads to an improvement of the detection accuracy. The number of identical synchronous patterns, which is required for the synchronization establishment, is referred to as a synchronization establishment determination number or a synchronization protection number. The reason why the protection is provided by detecting not a single but several synchronous patterns is that the detection of the synchronization establishment determination is made easier than with a longer synchronous pattern, and moreover, the same accuracy as that implemented with the synchronous pattern whose length is equivalent to the synchronization establishment detection number can be realized.
FIG. 1J exemplifies a signal pattern which causes synchronization in a pseudo manner. Here, the LSB data is extracted every 125 xcexcs cycle in a similar manner as in FIG. iF, and the extracted data is stored in a memory. Since the cycle of an F bit is 500 xcexcs, 4 sequences 1 through 4 are prepared as memory sequences. Data is stored in the respective memory sequences every 500 xcexcs. The portion where 4 or more identical F bit patterns, xe2x80x9c10xe2x80x9d in this case, are detected is recognized to be the position of an F bit. In FIG. 1J, however, synchronization is determined to be made in the memory sequence 3 in a pseudo manner in addition to the proper position of the F bit, that is, the position in the sequence 2. Since the two F bits are determined to exist, the detection is again made normally.
Provided next is the explanation about the operations using F bits, which are performed by the multiplexing device of the conventional type (a), by referring to FIGS. 1K, 1L, and 1M. FIG. 1K shows the state where a bidirectional path serving as 100a and 100b within a digital relay exchange 100 is not connected, and the coder and the decoder of each of the multiplexing devices 102 and 103 are connected by being inserted between a transmission line and the relay exchange 100.
FIG. 1L shows the state where one of the voice paths 100b of the digital relay exchange 100 is connected. In this state, an F bit is inserted in a coded signal by the multiplexing device 103, and is input to the multiplexing device 102 via the digital relay exchange 100. The multiplexing device 102 detects the input of the F bit as the synchronous bit for relay exchange switching, and switches the multiplexing device 102 itself to the relay exchange mode, that is, the state where its coder and decoder are disconnected. Because the voice path from the multiplexing device 102 to the multiplexing device 103, that is, the voice path 100a of the relay exchange 100 is yet to be connected on the side of the multiplexing device 103, the multiplexing device 103 detects no F bits. Accordingly, the multiplexing device 103 does not switch its operating mode to the relay exchange mode.
FIG. 1M shows the state where also the other of the voice paths 100a of the digital relay exchange 100 is connected. In this state, an F bit is inserted in the signal output from the multiplexing device 102, and the F bit is detected by the multiplexing device 103. As a result, also the multiplexing device 103 switches its operating mode to the relay exchange mode. Here, the operations for switching to the relay exchange mode are completed.
As described above, multiplexing devices on both sides of a relay exchange perform synchronization establishment, and their modes are immediately switched to the relay exchange mode. However, the upstream and downstream of a voice path are not always connected at the same time, depending on the specification of a relay exchange. The upstream may be connected after all of downstream paths are connected, or the downstream may be connected after all of upstream paths are connected.
Referring to FIGS. 1A and 1L. With the method for initially connecting an upstream, for example, the voice path 100b within the relay exchange 100 is initially connected, for example, as shown in FIG. 1L. With this method, the upstream is initially connected in order to allow a local exchange side to hear a second dial tone or a busy tone when control information reaches a connection destination or a relay exchange, and the downstream is not connected until all of relay exchanges receive the control information. Therefore, after the multiplexing device 102 is switched to the relay exchange mode, a considerable amount of time is required until the multiplexing device 103 is switched to the relay exchange mode.
In the meantime, with the method for initially connecting a downstream, the downstream path is firstly connected in order to transmit the control information such as a PB (Push Button) signal, etc. from a local exchange side, and an upstream is not connected until the connection of the downstream path is completed. This is because a called party does not exist until the completion of the connection, and noise, etc. is not caused to the local exchange. The multiplexing device 103 is firstly switched to the relay exchange mode, while the multiplexing device 102 requires a considerable amount of time to be switched to the relay exchange mode.
As a system for overcoming the above described problem that a considerable amount of time is required to switch one of the multiplexing devices to the relay exchange mode although the other of the multiplexing devices of a relay exchange has been switched to the relay exchange mode, the system using a multiplexing device of a conventional type (b) exists. With this system, two types of F bits, that is, first and second F bits are prepared. In the state where the bidirectional path is not connected in a relay exchange, each of multiplexing devices inserts the first F bit in the signal to be output, and outputs this signal to the relay exchange side. When one of the paths is connected within a relay exchange, one of the multiplexing devices receives the coded signal in which the F bit is inserted via the connected path. This multiplexing device does not yet switch its operating mode to the relay exchange mode at this time, replaces the first F bit inserted in the coded signal with the second F bit, inserts the second F bit in the coded signal, and outputs the signal to the relay exchange side. Since the other of the voice paths is yet to be connected at this time, the other of the multiplexing devices does not switch its operating mode.
When the path in the other direction is connected within the relay exchange, the multiplexing device at the connection destination detects the second F bit from the coded signal, establishes synchronization, and switches its operating mode to the relay exchange mode. At the same time, the multiplexing device replaces the first bit inserted in the coded signal with the second F bit, and outputs the signal to the relay exchange side. Since the voice path at the output destination is currently connected, the multiplexing device which receives this signal detects the second F bit from the input signal, establishes synchronization, and switches its operating mode to the relay exchange mode. In this way, both of the multiplexing devices switch their modes to the relay exchange mode almost at the same time, after the connections of the voice paths within the relay exchange are completed.
FIG. 1N is a time chart showing the operations for firstly connecting an upstream, which are performed by the multiplexing device of the conventional type (a), when an A side originates a call to a D side. When the paths within a digital relay exchange are not connected, the digital relay exchange outputs a no-tone code as will be described later. The coder included in the multiplexing device receives the PCM code to which the F bit as the no-tone code is not added.
When the call origination is notified from the A side, the digital relay exchange connects an upstream path ES from a line B of a TTC (The Telecommunication Technology Committee)-2- Mbps line to a line C. Then, a multiplexing device #2 receives the PCM signal to which the F bit is added by a decoder D1 of a multiplexing device #1, detects synchronization establishment, switches the multiplexing device #1 itself to the state where a coder C2 and a decoder D2 are not connected, that is, to the relay exchange mode, and enters a through state. Here, the call connection is completed.
Next, the digital relay exchange connects a downstream path ER from the line C to the line B. The multiplexing device #1 then receives the PCM signal to which the F bit is added by the decoder D2 of the multiplexing device #2, detects synchronization establishment, switches the multiplexing device #1 itself to the state where a coder C1 and the decoder D1 are not connected, that is, to the relay exchange mode, and enters a through state. Here, the connection between the A and D sides is completed.
In such a connection flow, voice signals cannot be transmitted for several seconds to several-tens of seconds from when the coder C2 and the decoder D2 enter the through state till when the coder C1 and the decoder D1 enter the through state, because of the mismatch between signal formats as will be described later.
FIG. 1O is a time chart showing the operations for firstly connecting a downstream, which are performed by the multiplexing device of the conventional type (a), when an A side originates a call to a D side.
When the call origination is notified from the A side, a digital relay exchange connects a downstream path ER from a line C to a line B. Then, a multiplexing device #1 receives the PCM signal to which an F bit is added by a decoder D2 of a multiplexing device #2, detects synchronization establishment, switches the multiplexing device #1 itself to the state where a coder D1 and a decoder D1 are not connected, that is, to the relay exchange mode, and enters a through state.
Next, the digital relay exchange connects an upstream path ES from the line B to the line C upon completion of the connection of the call. Then, the multiplexing device #2 receives the PCM signal to which the F bit is added by the decoder D1 of the multiplexing device #1, detects synchronization establishment, switches the multiplexing device #2 itself to the state where a coder C2 and the decoder D2 are not connected, that is, to the relay exchange mode, and enters the through state. Here, the connection between the A and D sides is completed.
In such a connection flow, voice signals cannot be transmitted for several seconds to several-tens of seconds from when the coder C1 and the decoder D1 enter the through state till when the coder C2 and the decoder D2 enter the through state, because of the mismatch between signal formats as will be described later.
As described above, the conventional multiplexing device includes the types (a) and (b). Since the conventional type (a) detects synchronization establishment with the F bit and simultaneously switches its operating mode to the relay exchange mode, a considerable amount of time is required to switch the multiplexing device on one side to the relay exchange mode although the multiplexing device on the other side has been switched to the relay exchange mode, depending on the kind of a relay exchange.
As described above, intermediate multiplexing devices only relay a voice signal without coding/decoding the signal in the relay exchange mode, so that a 1-link path is established between the multiplexing devices which are respectively closest to the telephones on call originating and terminating sides. If path connections within a relay exchange are not simultaneously made, some of the intermediate multiplexing devices are switched to the relay mode, while some of them continue to operate in the normal mode.
Assume that the multiplexing device 102 switches its operating mode to the relay exchange mode, while the multiplexing device 103 continues to operate in the normal mode as shown in FIG. 1L. In this case, the signal from the exchange C is coded by the multiplexing device 104, decoded by the multiplexing device 103, switched by the relay exchange 100, and is input to the multiplexing device 102. However, since the multiplexing device 102 is in the relay exchange mode, it does not code the signal and inputs the multiplexing device 10 unchanged. The multiplexing device 101 is continually in the normal mode. Therefore, the multiplexing device 101 decodes this signal and outputs it to the exchange A. As described above, the signal output from the relay exchange 100 is not coded but only decoded, which leads to a mismatch between the signal formats. This causes a serious problem in that a voice signal cannot be transmitted, and a call cannot be connected with a PB signal due to the disability of the transmission 1 of information although the information of a telephone number, etc. is attempted to be transmitted with the PB signal during the establishment of the call.
As one of the methods for overcoming this problem, the conventional type (b) is used. This type, however, uses the two F bit types, that is, the first and second F bits as F bit sequences. Therefore, an F bit inserting unit must insert these two types, while an F bit detecting unit must detect the respective types. As a result, the size of circuitry may become large.
Additionally, if the multiplexing device of the conventional (b) type is opposed to that of the conventional type (a), the conventional type (a) does not include the F bit inserting and detecting units which can process one of the two F bit types although the other of the F bit types can be inserted and detected within the conventional type (a). Accordingly, switching to the relay exchange mode cannot be made, so that the conventional types (a) and (b) cannot be opposed and used together.
An object of the present invention is to provide a multiplexing device which prevents signal formats from mismatching by switching to the relay exchange mode after verifying the connections of voice paths of a relay exchange, and can oppose the multiplexing device of a conventional type (a), with circuitry reduced in size by limiting the bit pattern of a signal used as an F bit to one type.
The multiplexing device according to the present invention comprises a transmitting unit, a receiving unit, a controlling unit, a first periodic signal inserting unit , a second periodic signal inserting unit, an operating mode switching unit, and a periodic signal inserting unit.
In a first aspect of the present invention, the transmitting unit bypasses the process for switching data received from a network side, and transmits the data to an exchange side. The receiving unit bypasses the process for the data, which is received from the exchange and is bypassed by the transmitting unit. The controlling unit controls the bypass operations of the data after verifying the connections on both sides of a bidirectional communication path by using a single pattern as a synchronous bit pattern.
In a second aspect of the preset invention, the first periodic signal inserting unit inserts a first periodic signal which periodically or intermittently includes one or more predetermined bit patterns, in a signal to be output to the multiplexing device on an opposing side, and outputs the signal to an exchange side, when the path from the multiplexing device on the opposing side is not connected. The second periodic signal inserting unit inserts a second periodic signal, which periodically or intermittently includes one or more bit patterns identical to the predetermined bit patterns or one or more different bit patterns in addition to the one or more predetermined bit patterns, in the signal to be output to the multiplexing device on the opposing side, and outputs the signal to the exchange side, from a time point at which the first periodic signal inserted by the first periodic signal inserting unit on the opposing side is detected within a signal input from the exchange after the path from the multiplexing device on the opposing side is connected. The operating mode switching unit switches its local multiplexing device to the relay exchange mode for relaying an input signal without coding or decoding it upon detection of the second periodic signal that the second periodic signal inserting means on the opposing side inserts in the signal input from an exchange, and relays the input signal.
In a third aspect of the present invention, the periodic signal inserting unit inserts a periodic signal which periodically or intermittently includes one or more predetermined bit patterns, in a signal to be output to the multiplexing device on an opposing side, and outputs the signal to an exchange side, from when the change from a disconnected state to a connected state of the path from the multiplexing device on an opposing side is detected based on the change from the voiceless to the voiced state of the signal input from the exchange. The operating mode switching unit switches its local multiplexing device to the relay exchange mode for relaying an input signal without coding or decoding it upon detection of the periodic signal that the above described periodic signal inserting unit on the opposing side inserts in the signal input from the exchange after the path from the multiplexing device on the opposing side is connected, and relays the input signal.
In a fourth aspect of the present invention, the first periodic signal inserting unit inserts a first periodic signal which periodically or intermittently includes one or more predetermined bit patterns and a bit having a predetermined value, in the signal to be output to the multiplexing device on an opposing side, and outputs the signal to an exchange side, when the path from the multiplexing device on the opposing side is not connected. The second periodic signal inserting unit inserts a second periodic signal which periodically or intermittently includes one or more predetermined bit patterns and the bit having the value obtained by inverting the predetermined value of the bit, which are included in the above described first periodic signal, in the signal to be output to the multiplexing device on the opposing side, and outputs the signal to the exchange side, from when one or more predetermined bit patterns included in the first periodic signal that the first periodic signal inserting unit inserts in the signal input from the exchange are detected after the path from the multiplexing device on the opposing side is connected. The operating mode switching unit switches its local multiplexing device to the relay exchange mode for relaying an input signal without coding or decoding it upon detection of the inversion of the predetermined value of the bit included in the second periodic signal that the second periodic signal outputting unit on the opposing side inserts, within the signal input from the exchange.